/m15-anti-pattern

name: m15-anti-pattern
description: "Use when reviewing code for anti-patterns. Keywords: anti-pattern, common mistake, pitfall, code smell, bad practice, code review, is this an anti-pattern, better way to do this, common mistake to avoid, why is this bad, idiomatic way, beginner mistake, fighting borrow checker, clone everywhere, unwrap in production, should I refactor, 反模式, 常见错误, 代码异味, 最佳实践, 地道写法"
user-invocable: false

Anti-Patterns

> **Layer 2: Design Choices**

Core Question

**Is this pattern hiding a design problem?**

When reviewing code:

• Is this solving the symptom or the cause?
• Is there a more idiomatic approach?
• Does this fight or flow with Rust?

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Anti-Pattern → Better Pattern

| Anti-Pattern | Why Bad | Better | |--------------|---------|--------| | `.clone()` everywhere | Hides ownership issues | Proper references or ownership | | `.unwrap()` in production | Runtime panics | `?`, `expect`, or handling | | `Rc` when single owner | Unnecessary overhead | Simple ownership | | `unsafe` for convenience | UB risk | Find safe pattern | | OOP via `Deref` | Misleading API | Composition, traits | | Giant match arms | Unmaintainable | Extract to methods | | `String` everywhere | Allocation waste | `&str`, `Cow<str>` | | Ignoring `#[must_use]` | Lost errors | Handle or `let _ =` |

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Thinking Prompt

When seeing suspicious code:

1. **Is this symptom or cause?**

• Clone to avoid borrow? → Ownership design issue
• Unwrap "because it won't fail"? → Unhandled case

2. **What would idiomatic code look like?**

• References instead of clones
• Iterators instead of index loops
• Pattern matching instead of flags

3. **Does this fight Rust?**

• Fighting borrow checker → restructure
• Excessive unsafe → find safe pattern

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Trace Up ↑

To design understanding:

"Why does my code have so many clones?"
    ↑ Ask: Is the ownership model correct?
    ↑ Check: m09-domain (data flow design)
    ↑ Check: m01-ownership (reference patterns)

| Anti-Pattern | Trace To | Question | |--------------|----------|----------| | Clone everywhere | m01-ownership | Who should own this data? | | Unwrap everywhere | m06-error-handling | What's the error strategy? | | Rc everywhere | m09-domain | Is ownership clear? | | Fighting lifetimes | m09-domain | Should data structure change? |

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Trace Down ↓

To implementation (Layer 1):

"Replace clone with proper ownership"
    ↓ m01-ownership: Reference patterns
    ↓ m02-resource: Smart pointer if needed

"Replace unwrap with proper handling"
    ↓ m06-error-handling: ? operator
    ↓ m06-error-handling: expect with message

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Top 5 Beginner Mistakes

| Rank | Mistake | Fix | |------|---------|-----| | 1 | Clone to escape borrow checker | Use references | | 2 | Unwrap in production | Propagate with `?` | | 3 | String for everything | Use `&str` | | 4 | Index loops | Use iterators | | 5 | Fighting lifetimes | Restructure to own data |

Code Smell → Refactoring

| Smell | Indicates | Refactoring | |-------|-----------|-------------| | Many `.clone()` | Ownership unclear | Clarify data flow | | Many `.unwrap()` | Error handling missing | Add proper handling | | Many `pub` fields | Encapsulation broken | Private + accessors | | Deep nesting | Complex logic | Extract methods | | Long functions | Multiple responsibilities | Split | | Giant enums | Missing abstraction | Trait + types |

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Common Error Patterns

| Error | Anti-Pattern Cause | Fix | |-------|-------------------|-----| | E0382 use after move | Cloning vs ownership | Proper references | | Panic in production | Unwrap everywhere | ?, matching | | Slow performance | String for all text | &str, Cow | | Borrow checker fights | Wrong structure | Restructure | | Memory bloat | Rc/Arc everywhere | Simple ownership |

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Deprecated → Better

| Deprecated | Better | |------------|--------| | Index-based loops | `.iter()`, `.enumerate()` | | `collect::<Vec<_>>()` then iterate | Chain iterators | | Manual unsafe cell | `Cell`, `RefCell` | | `mem::transmute` for casts | `as` or `TryFrom` | | Custom linked list | `Vec`, `VecDeque` | | `lazy_static!` | `std::sync::OnceLock` |

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Quick Review Checklist

• [ ] No `.clone()` without justification
• [ ] No `.unwrap()` in library code
• [ ] No `pub` fields with invariants
• [ ] No index loops when iterator works
• [ ] No `String` where `&str` suffices
• [ ] No ignored `#[must_use]` warnings
• [ ] No `unsafe` without SAFETY comment
• [ ] No giant functions (>50 lines)

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Related Skills

| When | See | |------|-----| | Ownership patterns | m01-ownership | | Error handling | m06-error-handling | | Mental models | m14-mental-model | | Performance | m10-performance |